The invention is in the field of apparatus for irradiating articles, such as coatings for wires, cables or the like, to improve the quality of the irradiated articles, and more particularly is directed to an apparatus for achieving a more nearly uniform irradiation of the entire peripheral surface of such articles.
It is well known in the art to irradiate materials having a high molecular weight, such as polyethylene and polyvinylchloride, to improve the quality thereof. Typically, electron beam irradiation is used, and the process has particular utility for improving the coatings of electric wires, cables, tubes and the like. When applied to such devices one of the problems encountered is how to uniformly irradiate the coatings from a charged particle beam emanating from one side of the article as the article is pulled through the field scanned by the beam. One reason why this is a problem is because the penetration depth of the beam is inversely proportional to the beam energy. For example the penetration depth of an electron beam of 2MeV is only 1 cm at most in water and about 0.1 cm in copper having a density of 8.9 g/cm.sup.3. Therefore, when an article such as an electric cable is irradiated by an electron beam emanating from one side thereof, the back side will be shielded by the core conductor of copper and the portion of the coating just behind the conductor will receive no radiation.
In order to guide the electron beam to the back or far side of the cable, it is usual to employ a pair of magnetic structures to deflect electron beams incident normally thereto toward the corresponding halves of the cable. The magnetic structures, each of which includes a magnet and a pair of pole pieces for forming a suitable magnetic field therebetween, are arranged in parallel such that the polarities of the pole pieces and the magnets are opposite in direction, to produce a pair of oppositely polarized, parallel magnetic fields. The cable is passed through a space between the magnetic structures, with half of the cable subjected to one polarity magnetic field and the other half to an opposite polarity magnetic field.
One example for carrying out latter process is disclosed in Japanese Patent Publication No. 435/1960. However the apparatus described does not take into consideration magnetic leakage at the end portions of the magnet (the so-called fringing effect) and the corresponding electron energy loss. Also, with path travelled through the magnetic fields by the electron beams is quite long resulting in an energy loss and therefore current decrease of the electron beam. Consequently the apparatus has not been practical.
As another example, where the article to be irradiated is a tube of a large diameter, e.g. 60 mm., it is usual to support the tube rotatably by a conveyer and rotate it under a unidirectional irradiation. In such case, assuming the tube is of for example, polyethylene which exhibits a good insulator property and is irradiated with, for example. 2MeV beam, the electrons are accumulated excessively in portions of the wall of the tube around 2mm depth and they are discharged subsequently. This is referred to as Lichtenberg phenomenon and the result thereof is referred to as Lichtenberg discharge.